Ouch, This to me sounds bad. It seems that if someone left with an extra stick, there would be no way you could not know, As the price is enough to know if you had invested in a whole extra stick.
and a problem I foresee is if someone unknowingly took your stick, thinking it was theirs, What happened to their stick?
I hope it works out for you, Because I would kill a mother fucker if I found they stole my TE.
Do you have the model number on the back of the TE to compare? Without that info, and since your stick is stock, I would think it would be hard to prove ownership.